High-hole mobility polycrystalline Ge on an insulator formed by controlling precursor atomic density for solid-phase crystallization

Abstract

High-carrier mobility semiconductors on insulators are essential for fabricating advanced thin-film transistors, allowing for three-dimensional integrated circuits or high-performance mobile terminals. We investigate the low-temperature (375–450 °C) solid-phase crystallization (SPC) of Ge on a glass substrate, focusing on the precursor conditions. The substrate temperature during the precursor deposition, T d, ranged from 50 to 200 °C. According to the atomic density of the precursor and the T d dependent SPC properties, the precursor conditions were determined by three regimes: the low-density regime (T d < 100 °C), high-density regime (100 ≤ T d ≤ 125 °C), and nucleation regime (T d > 125 °C). The use of the precursor in the narrow high-density regime enabled us to form SPC-Ge with a hole mobility of 340 cm2/Vs, the highest value among semiconductor thin films grown on insulators at low temperature (<900 °C). The origins of the high hole mobility were determined to be both a large grain size (5 µm) and a low energy barrier height (6.4 meV) for the grain boundary. The findings from and knowledge gained in this study, that is, the influence of the precursor conditions on subsequent crystal growth, will be universal and applicable to various materials.

Sb-doped crystallization of densified precursor for n-type polycrystalline Ge on an insulator with high carrier mobility

“…To further improve l n , we investigated the effects of T g and t. Figure 4(a) shows that the grain size increases by lowering T g , reflecting the decrease in the nucleation frequency. 9,30 Conversely, the grain size is almost constant with respect to t. As shown in Fig. 4(b), generally, the grain size of undoped Ge decreases with increasing t. This behavior was explained as the bulk nucleation occurring in thick films while interfacial nucleation remained predominant in thin films.…”

“…29 We recently found that the atomic density of amorphous Ge (a-Ge) significantly influenced subsequent SPC. [30][31][32] By using a densified a-Ge on a GeO 2 underlayer, we fabricated a poly-Ge layer with a hole mobility of 620 cm 2 /V s, 33 which greatly exceeds that of bulk-Si (430 cm 2 /V s). These achievements initiated the prospect of the development of inversion-type poly-Ge n-MOSFETs that surpass Si-MOSFETs.…”

“…This phenomenon has been explained as the effect of densifying an amorphous precursor. 30 . For C Sb ¼ 1.6 Â 10 20 and 5.0 Â 10 20 cm À3 , large grains were also observed at T d ¼ 50 and 175 C, respectively.…”

“…1(l)], where the as-deposited undoped Ge layer has dense nuclei. 30 Consequently, Sb doping in the Ge precursor has a large influence on the nucleation and lateral growth in subsequent SPC, allowing for large grain growth.…”

“…1). This behavior is likely because the larger grain size provides the lower defect-induced acceptors 30,31 and/or the less Sb segregation at grain boundaries. Figure 3(b) shows that n is saturated at around C Sb ¼ 2.8 Â 10 20 cm À3 for each T d .…”